Crystal-Chemical Considerations in the Choice of Matrices for REE-Actinides

Abstract

Light rare earth (REE) titanates, such as REE₂TiO₅, REE₂Ti₂O₇, and REE₄Ti₉O₂₄, are potential matrices for the REE-actinide fraction of high-level waste from the reprocessing of spent nuclear fuel. The data on the “solubility” of impurity elements (zirconium, uranium, and calcium) in these phases are summarized. The structures considered demonstrate limited isomorphism with respect to these elements, according to the reaction 2REE³⁺ = Ca²⁺ + U⁴⁺, which is common for natural minerals and their synthetic analogues. The reasons for the low “solubility” of these impurities in the REE titanates are considered. The role of the crystal-chemical factor in the selection of matrices for the immobilization of the REE-actinide fraction is analyzed.

Russia is implementing two-component nuclear power based on both thermal-neutron and fast-neutron reactors with reprocessing of spent nuclear fuel [1]. Radioactive waste, including high-level waste (HLW), originates from the nuclear fuel cycle. The development of the methods of handling long-lived transuranium actinides (Pu and minor actinides, Np, Am, Cm) is one of the key tasks in improvement of nuclear power safety. Minor actinides can be extracted from the HLW in the form of a REE-MA fraction (MA = Am, Cm) and incorporated in the crystalline phases with low solubility in water for subsequent geological disposal [2]. The REE-MA fraction mostly consist of large lanthanides (La, Ce, Pr, Nd, Sm); Am and Cm account for about 10 wt % [3]. Crystalline REE titanates and zirconates are the most promising matrices for the REE-MA fraction of SPF HLW [1, 2, 4]. Most studies have been devoted to oxides with the structures of pyrochlore and fluorite [2, 4, 5]. Fewer works are available on the properties of the REE titanates. They mainly concern the influence of the REE cations of the structure type of the REE₂TiO₅ and REE₂Ti₂O₇ phases and the study of their resistance to ion irradiation [6, 7].

Nd compounds are of great interest in the search for REE-MA matrices because Nd³⁺ is a chemical analogue (imitator) of Am³⁺ and Cm³⁺. The following phases occur in the Nd₂O₃–TiO₂–ZrO₂ system [8]: pyrochlore Nd₂(Ti,Zr)₂O₇ (hereafter, LnTZ), TiO₂ (T, rutile), ZrTiO₄ (ZT, shrilankite) and tetragonal ZrO₂ (Z_t). Nd-titanates are represented by Nd₂TiO₅ (LnT), Nd₂Ti₂O₇ (LnT₂), Nd₂Ti₄O₁₁ (LnT₄), and Nd₄Ti₉O₂₄ (Ln₂T₉). Among other REE titanate and zirconate systems, La₂O₃–TiO₂–ZrO₂ [9], Y₂O₃–TiO₂–ZrO₂ [10], and Nd₂O₃–TiO₂ [11] were studied. The identity of Nd₂Ti₄O₁₁ and Nd₄Ti₉O₂₄ were proven, and the occurrence of Nd₂Ti₃O₉ (LnT₃) was established in [11].

The La₂О₃–TiО₂–ZrО₂ system contains the following phases (Fig. 1b): La₂TiO₅ (LnT), La₄Ti₃O₁₂ (Ln₂T₃), La₂Ti₂O₇ (LnT₂), La₄Ti₉О₂₄ (Ln₂T₉), La₂Zr₂О₇ (LnZ₂), ZrTiО₄ (ZT), ZrO₂ (Z), and TiO₂ (T). The phase sets in the systems with Nd₂O₃ and La₂O₃ are similar. However, the phase relations in the systems considered are different, mostly due to the much smaller area of the La–Zr-pyrochlore (LnZ₂) solid-solution field. Weak variations in the chemical composition are a specific feature of Nd and La titanates. The Ti/REE ratio in the phases is close to stoichiometric values, which is manifested in the narrow stability fields in the diagrams. Less than 2 mol % of ZrO₂ dissolves in the LnT₂, Ln₂T₃, and LnT phases at 1350^оС [9], while Ln₂T₉ contains up to 4 mol % of ZrO₂ (Table 1). La₂Zr₂О₇ (LnZ₂) contains up to 35 mol % of La₂O₃ and 69 mol % of ZrO₂, while the ideal stoichiometry suggests 33 and 67 mol % of La₂O₃ and ZrO₂, respectively. Isomorphic substitutions of La³⁺ into ZrO₂ and ZrTiО₄ are limited by 1 mol % of La₂O₃. High contents (12–14 mol %) of ZrO₂ in (Ti,Zr)O₂ and TiO₂ in (Zr,Ti)O₂ were observed. The Zr/Ti ratio in ZrTiO₄ varies in a wide range (1.4–0.9). Substitution of large REE³⁺ (La, Nd) for Y³⁺ leads to the transformation of the phase diagram (Fig. 1c). The Y₂Ti₂O₇ displays a cubic symmetry (pyrochlore like) and a large solid solution field of (Zr,Y)O_2 – x fluorite appears in the phase diagram. Oxides TiO₂, ZrО₂, and ZrTiО₄ are preserved. The Y₂TiO₅ turns to a cubic modification (sp. group $Fm\bar {3}m$), while Nd₂TiO₅ and La₂TiO₅ belong to the orthorhombic symmetry (sp. group Pnma).

Fig. 1.

NdO_1.5–TiO₂–ZrO₂ (а, 1450°С), La₂O₃–TiO₂–ZrO₂ (b, 1350°С), and YO_1.5–TiO₂–ZrO₂ (c, 1500°С) systems: 1 and 2, pyrochlore and fluorite fields [8–10].

Table 1. Chemical composition of the phases (mol %) in the La₂O₃–TiO₂–ZrO₂ system according to [9]

Variations of the REE³⁺ radii in the REE₂О₃–TiO₂ and REE₂О₃–ZrO₂ lead to polymorphic transitions. The REE₂Ti₂O₇ and REE₂Zr₂O₇ compounds form three structure types—fluorite, pyrochlore, and perovskite-like monoclinic titanate [3, 5–7]. The pyrochlore stability field falls to the Shannon radii ratio REE/Ti(Zr) from 1.46 to 1.78. Below 1.46 the fluorite structure type is stable, and above 1.79 the perovskite-like structure type is stable. Titanates of heavy REEs (Ln₂Ti₂O₇, Ln = Sm–Yb, Y) and zirconates of light REEs (Ln₂Zr₂O₇, Ln = La–Gd) belong to the pyrochlore type of structure. The oxide REE₂TiO₅ displays an orthorhombic symmetry (REE = La–Sm), cubic symmetry (REE = Er–Lu, Sc), and hexagonal symmetry (REE = Ho, Y) [7]. The stability of the structure type with respect to the chemical composition is an important property of the matrices. The appearance of a phase with high solubility in water during the synthesis reduces the performance of the matrices. Previously we studied the distribution of the Ca, Zr, and U impurities in the REE titanates [12–16]. The samples were synthesized by cold pressing and sintering (CPS), cold crucible inductive melting (CCIM), and melting in Joule-heated furnace and crystallization (MC) methods. Tables 2–4 and Fig. 2 list the results of our experiments. A new series of U-bearing samples (NTС-1, NTС-2, NTU-4, NTZ-1, NTZ-2, NTZ-3) was studied.

Table 2. Chemical composition, experiment details, and phase composition according to XRD

Table 3. Chemical compositions of the REE–U samples “3b” and “4”

Table 4. Normalized compositions (on 100 wt %) of the U-bearing samples.

Fig. 2.

BSE images of (a) LT2 (1, LnT₂; 2, Ln₂T₉); (b) 3b (1, LnT₂; 2, UT₂, brannerite; 3, (Ln,U)O_2 – x; 4, REE Ti-silicate); (c) 4 (1, Ln₂T₉; 2, UT₂, brannerite; 3, (Ln,U)O_2 – x; 4, REE Ti-silicate); (d); MPM-2 (1, T, rutile; 2, Ln₂T₉), (e) NTC-2 (1, pyrochlore, 2, (Ln,U)O_2 – x); (f) NTU-4 (1, LnT₂; 2, Ln₂T₉; 3, (Ln,U)O_2 – x). Black dots, pores. Scale bar (а, d–f) 50, (b) 20, and (c) 10 microns. The chemical compositions of the phases are listed in Tables 3 and 4.

The observed significant variations of the phase chemical compositions with respect to REE³⁺ (Table 3) are due to the fact that lanthanide titanates (Ln³⁺ = La, Ce, Nd) of the same stoichiometry (LnT, LnT₂, Ln₂T₉) are isostructural. However, contrast to the other REE phases (pyrochlore, zirconolite, monazite, brannerite, etc.), titanates have low solubility for tetravalent (U, Zr) and divalent (Ca) elements.

The ZrO₂ content is 2.0 wt % in Ln₂T₉ (sample MPM-2) and 8.5 wt % in rutile. The U content is 1.3 wt % in Ln₂T₉ (Table 3) and is below the detection limit (0.3–0.5 wt %) in LnT and LnT₂ (Tables 3 and 4). The highest contents of U and Zr (>10 wt %) were found in pyrochlore (Table 4). Those peculiarities of the chemical compositions are due to the crystal chemical properties of the phases considered.

REE titanates and zirconates adopt several structure types. In most of the cases, Ti and Zr ions are six coordinated with O ions forming an octahedron. The dominant coordination numbers of large REEs (La–Sm) are VII (monocapped trigonal prism, truncated cube), VIII (cube, distorted cube, bicapped antiprism), and IX (tricapped trigonal prism). With a decrease in the REE³⁺ radii, the occurrence of coordination number VI increases. Description of the crystal structures of Nd³⁺ oxides is given in Table 5 and Fig. 3.

Table 5. Crystal-chemical features of the Nd-phases in the Nd₂O₃–TiO₂–ZrO₂ system

Fig. 3.

Crystal structures of the (а) Nd₂Zr₂O₇, (b) Nd₂Ti₂O₇, (c) Nd₂TiO₅, and (d) Nd₄Ti₉O₂₄. Ti and Zr polyhedra are green, and Nd polyhedral are brown.

The Nd₂Zr₂O₇ belongs to cubic symmetry (Fig. 3a), space group $Fd\bar {3}m$. The unit cell contains eight formula units. The Nd coordination polyhedron is scalenohedron (distorted cube) with six equidistant O ions and two O ions at a larger distance. The Zr ion centers the trigonal antiprism (distorted octahedron). The pyrochlore structure can be considered as the framework of the BO₆ corner-linked octahedra with AX₂ chains filling the interstices of the octahedral framework. The pyrochlore structure is a derivative of the fluorite structure type AO2 (sp. group $Fm\bar {3}m$).

The Nd₂Ti₂O₇ crystal structure is a derivative of the perovskite structure (Fig. 3b). The corner linked TiO₆ octahedra form slabs four octahedra thick (~12 Å) along the directions a and b. The monocapped trigonal prisms NdO₇ are in-between slabs. Tricapped trigonal prisms NdO₉ fill the interstices of the octahedral slabs. Bicapped prisms NdO₈ occur both inside and in-between slabs.

The Nd₂TiO₅ crystal structure (Fig. 3c) comprises a framework of edge-sharing monocapped trigonal prisms LnO₇ and chains of vertex sharing TiO₅ square pyramids along the [010] direction.

The Nd₄Ti₉O₂₄ oxide has the most complex structure. It consists of a TiO₆ octahedral framework, the cavities of which are filled with Nd ions (Fig. 3d). Nd ions occupy three independent positions in the structure: Nd(1) centers the distorted square antiprism, Nd(2) centers the octahedron, Nd(3) centers a distorted square prism. The Nd(1)O₈ polyhedra linked by common vertices and edges form layers parallel to the (110) plane. The Nd(3) polyhedra linked by common edges with Nd(1) layers form complex secondary layers (17.5 Å thick) the cavities of which are filled with isolated Nd(2) octahedra.

The coordination polyhedra of Zr⁴⁺ and Ti⁴⁺ are octahedra (coordination number equals VI) in all the phases considered, with the exception of Nd₂TiO₅, where the Ti⁴⁺ coordination number equals V and the shape of the coordination polyhedron is a square pyramid. This explains the different isomorphism of elements in the phases considered, i.e., a wide field of the pyrochlore solid solution and narrow fields of phases with ratios of elements close to their nominal stoichiometry in other cases. The low solubility of Ca, Zr, and U is typical for the phases with odd Nd coordination numbers (VII and IX). It is worth noting that part of the Nd ions is eight-coordinated in Nd₄Ti₉O_24. Probably that is the reason for the increased content of U and Zr in Nd₄Ti₉O₂₄ relative to Nd₂Ti₂O₇ and Nd₂TiO_5.

Narrow stability fields of La³⁺ and Nd³⁺ titanates (Figs. 1a, 1b) contrast with the wide composition variations of pyrochlore, zirconolite, and brannerite, which are also considered as matrices for the REE-actinide fraction. Zirconate and titanium–zirconate pyrochlores have a high isomorphic capacity for actinides in oxidation states other than 3+ [3–5, 20]. The Zr position of pyrochlore Nd₂Zr₂O₇ can host up to 20 at. % U, and the Nd position can host from 10 to 20 at. % U and Th. The wide ranges of compositions of zirconolite and brannerite are due to the peculiarities of their crystal structures [3, 4]. Zirconolite CaZrTi₂O₇ (Fig. 4a) adopts a monoclinic symmetry (C2/m), and the Ca and Zr coordination numbers are VIII and VII, respectively. The REE and actinide cations can replace Ca, Zr, and Ti by three mechanisms. The first of them is heterovalent: Ca²⁺ + Zr⁴⁺ → 2M$_{1}^{{3 + }}$ (M₁ = lanthanides (Ln) and actinides (An)). At a high content of M₁ cations, this structure transforms into (M₁)₂Ti₂O₇ pyrochlore. The second variant of heterovalent isomorphism Ca²⁺ + Ti⁴⁺ → M$_{1}^{{3 + }}$ + M$_{2}^{{3 + }}$ (M₁ = Ln and An = Pu, Am, Cm; M₂ = Al, Fe). In addition, isovalent substitution of Zr⁴⁺ for M$_{3}^{{4 + }}$ cations (M₃ = Ce, An) is possible. In zirconolite, actinides (U, Np, Pu) occur in the Ca and Zr positions in an amount of up to ~0.3 atoms per formula unit. When Zr is completely replaced by actinides, pyrochlore CaAn⁴⁺Ti₂O₇ is formed. Brannerite crystallizes in a monoclinic symmetry (sp. group C2/m). The U and Ti cations in the structure of brannerite are located in the centers of octahedra. The TiO₆ octahedra linked by vertices and edges form layers that are parallel to the (001) plane. The columns of UO₆ octahedra going along the b axis (Fig. 4b) link the layers together. Large positions are occupied by (Ce, Th, U, Np, Pu)⁴⁺ or cations of a higher oxidation state (Np⁵⁺, U⁵⁺/U⁶⁺) with simultaneous incorporation of Ca²⁺ and REE³⁺ into the structure to compensate for the charge balance, for example, by the heterovalent scheme: Ca²⁺ + U⁶⁺ → 2U⁴⁺ or REE³⁺ + U⁵⁺ → 2U⁴⁺.

Fig. 4.

Crystal structures of (a) zirconolite CaZrTi₂O₇ (Ti layers are gray, Ca and Zr atoms are blue and green, respectively) and (b) brannerite UTi₂O₆ (U octahedra (yellow) forms columns parallel to the b axis, Ti octahedra (gray) form layers along the (001) plane. U octahedra are linked with Ti octahedra by vertex sharing.

CONCLUSIONS

Using the phases of Nd (an imitator of Am and Cm), variations in the composition and crystal chemistry of possible REE-MA matrices in the Nd₂O₃–TiO₂–ZrO₂ system are considered. The system contains, in the order of decrease in the stability fields in the diagram (Fig. 1a), the following phases: Nd₂(Zr,Ti)₂O₇, Nd₄Ti₉O₂₄, ZrTiO₄, (Zr,Ti)O₂, (Ti,Zr)O₂, Nd₂Ti₂O₇, and Nd₂TiO₅. Potential matrices for the REE-MA fraction are REE₂(Zr,Ti)₂О_7–х with the pyrochlore structure, Nd₄Ti₉O₂₄, Nd₂Ti₂O₇ (perovskite), and Nd₂TiO₅. Pyrochlore Nd₂(Ti,Zr)₂O₇ has the largest stability field, which includes 10 wt % or more of U and Zr. The “solubility” of Ca, Zr, and U in Nd-titanates is low: from 2–3 wt % in Nd₄Ti₉O₂₄ to below the detection limit (0.3–0.5 wt %) in Nd₂TiO₅ and Nd₂Ti₂O₇. At an increased content of admixtures, U and REE oxide, U titanate (brannerite), and Ca and Zr titanate (zirconolite) appear in the matrices. The last two compounds are highly stable in solutions, and their appearance will not lead to a deterioration in the insulating properties of matrices with respect to REE-MA. In the REE₂O₃–TiO₂ and REE₂O₃–ZrO₂ systems, as the REE³⁺ radius decreases, the structure of REE₂Ti₂O₇ compounds changes from monoclinic perovskite-like to cubic pyrochlore: the polymorphic transition boundary is between Nd and Sm. The REE₂Zr₂O₇ phases adopt two related structure types—pyrochlore and fluorite. The first appears at a large difference between the REE³⁺ and Zr⁴⁺ radii (R_REE/R_Zr > 1.46). As the difference in the ion radii decreases, the pyrochlore structure transforms into an anion-deficient fluorite (REE, Zr)₄O₇ structure. The phase transition boundary goes along Gd³⁺: zirconates of lighter and larger REEs of the Ce group belong to the pyrochlore structure type, while heavy and smaller REEs of the Y group adopt a fluorite-type structure. The situation is more complicated in the case of REE₂TiO₅ compounds: the REE phases from La to Sm adopt an orthorhombic symmetry, from Er to Lu and Sc they have a cubic symmetry, and the REE phases from Eu to Ho and Y are hexagonal.

Knowledge of the crystal chemical features of REE phases makes it possible to control the phase composition of the matrices. Admixtures (Zr, Fe, residual amounts of Pu, U) can either be present in the REE-MA fraction or can be specially introduced (CaO, Fe₂O₃, Al₂O₃) into the charge before the matrix is synthesized. As a result, along with REE-MA titanates, it is possible to obtain phases with the structures of pyrochlore, brannerite, and zirconolite, which do not deteriorate the properties of the matrix due to their high corrosion resistance in water.